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Latin arduus
← Welsh ardd (cognate)
← Ancient Greek ὀρθός (cognate)
← Proto-Indo-European *h₃erdʰ- (root)
← Proto-Indo-European *h₃erdʰ- (derived)
← Proto-Indo-European *h₃erdʰ- (mention)
← Proto-Indo-European *-wós (mention)
← Proto-Italic *arðwos (derived)
← Latin -uus (mention)
← Latin arbor (cognate)
← Sanskrit ऊर्ध्व (cognate)
← Old Irish ard (cognate)

→ English arduous (mention)
→ Catalan ardu (derived)
→ Spanish arduo (borrowed)
→ French ardu (mention)
→ Irish ard (cognate)
→ Scottish Gaelic àrd (cognate)
→ Galician arduo (borrowed)
→ Ancient Greek ὀρθός (cognate)
→ Manx ard (cognate)
→ Italian arduo (borrowed)
→ Latin arbor (mention)
→ Latin arduitas (affix)
→ Latvian rast (cognate)
→ Sanskrit ऊर्ध्व (cognate)
→ Old Irish ard (cognate)
→ Tocharian B ārte (cognate)